1 Introduction

Processed fine solid waste (tailings) slurry contains the entire spectrum of uranium series radionuclides. The disposal is normally carried out within a stable natural impoundment with site specific engineered control measures and surveillance systems. Due to the presence of moderate amount of activity from uranium series radionuclides, the site can be considered as potential source of exposure for the population residing around the facility [1]. A comprehensive radiological monitoring plan is worked out for evaluating the radiological impact of such discharges. The monitoring plans are part of the regulatory appraisal program of the facility consisting of radiological surveys in the surrounding area and the estimation of radionuclides of concern in the environmental samples. Environmental radiological surveillance around the tailings impoundment facility includes ambient gamma radiation, atmospheric radon activity concentration and long-lived alpha activity in respirable dust particles. Radiation dose to members of the public is assessed using these radiological parameters. Radon is a daughter product of 226Ra which is present in uranium mill tailings responsible for higher concentration of radon over and around the tailings disposal facility. Therefore, monitoring and assessment programs are undertaken to ensure that the radiological conditions are markedly deviating from the background or the public exposure surrounding the facility is not exceeding beyond the approved regulatory levels. The findings are quite helpful in reviewing the integrity of the waste management system vis a vis required interventions for the long term protection. The uranium mineralized region has a tropical climate with extended summer from April to June and a relatively short winter from December to February. Maximum temperature may exceed 40 °C during day time and 18 °C during the night. Maximum temperature during the winter is normally lower than 30 °C during the day and 10 °C during the night. The area witness more than 1400 mm annual average rainfall mostly due to the South-west monsoon extending from mid-June to mid-September [2]. The region hosts two underground uranium mines at Turamdih and Mohuldih and an opencast uranium mines at Banduhurang. The centralized ore processing unit at Turamdih receives the ore feed from all the three mines for the recovery of uranium ore concentrate or yellow cake [3]. The hydrometallurgy of ore is carried out in sulphuric acid medium using pyrolusite oxidant, followed by ion exchange separation and product recovery. The solid waste produced is separated in coarse and fine fractions, later discharged into an engineered impoundment system with natural hill barriers on two sides [4]. Typically the fine fraction discharged at tailings pond has a size distribution of < 75 micron. The area (Fig. 1) is hosting large quantity of process fine tailings stored during the past decade which can be a potential source of exposure for public residing around. Periodic radiological monitoring has been carried out around adjacent to and around the facility since inception of the discharges during previous decade.

Fig. 1
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Map of the study area

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Present paper is an overview of the radiological monitoring program for tailings disposal site of the low grade uranium ore processing plant at Turamdih Jharkhand. The findings in this paper consist of results of gamma level, atmospheric radon and estimated long lived alpha activity in the respirable dust.

2 Tailings Pond at Turamdih

The design feature of tailings pond at Turamdih is based on the latest state of art technology, situated in a geologically stable area with a number of additional control measures in place such as embankment, linings, covers and water control structures like spill way and drainage systems. Two parallel hills act as natural barrier for long term storage. Tailings pond is constructed using downstream method in a sloping valley with dams at eastern side and western side of the impoundment. A compacted impervious clayey material of very low permeability ~ 1 × 10−9 m s−1 has been used as a random fill for prohibiting the contaminant migration. Runoff water channels and garland drains are constructed around the impoundment to avoid the collection of excess water inside. The design features duly accounts the extreme values of the rainfall based on very long return period (of the order of 1000 years). Overflowing effluent from the routine discharges after settling of slurry is pumped at the Effluent Treatment Plant (ETP) for the removal of radionuclides prior to releases into the aquatic system through a delay storage pond [5, 6].

3 Radiological Hazard Consideration

As stated earlier, atmospheric radon and external gamma level can be considered as a potential source of exposure for the population residing around the facility. The exhaled radon is subjected to large atmosphere dilution thereby reducing the level at background within a very short distance. The gamma level change without a substantial physical transport of tailings material in the nearby area is unlikely. Discharges at the tailings pond are in the slurry form accordingly the dispersion of the tailings particulate into the atmosphere is unlikely. Despite very low anticipated variations radiological environmental monitoring has been the part of practice and is discussed in the subsequent sections.

4 Materials and Methods

Systems with very high sensitivity are used for the estimation of low atmospheric radon concentration anticipated in the environment. A professional radon monitor, Alpha Guard (Bertin Technologies, France) with Ionisation chamber detector and 3D alpha spectrometry is used for this purpose. Gaseous radon diffuses into the detector and after equilibration (more than one hour at a specified location) with the ambient condition the radon concentration and meteorological parameters are recorded. In a step change mode for radon concentration measurement, the system reaches 30% of the new value after 10 min, 70% after 20 min and 90% after 30 min. Two subsequent readings are taken and the last hour average is taken as the reading at the site. Concentration as low as 3 Bq m−3 can be measured using this device. Along with this, gamma radiation level was measured in µSvh−1 at one meter above the ground level using pre calibrated direct reading 1″ × 1″NaI(Tl) gamma spectroscopic based Isotope Identifinder [4]. In the respirable dust alpha activity has been monitored using the air sampler. The samples of air are drawn through a filter media. The APM 550 MFC ENVIROTECH sampler consists of omni-directional air inlet, impactor for particles larger than 10 micron and a PM2.5 impactor separated by a length of tube. The air activity (gross alpha) has been evaluated after a substantial delay of five days allowing the decay of short lived progeny of radon and thoron [1].

Quality assurance (QA)/ quality control (QC) features were adhered for ensuring reliability of the measurements. Radon monitoring equipments were calibrated in the laboratory using a calibration chamber having the concentration of 222Rn. A provision was made to use the system both for active and passive measurements. Active measurements were done using the scintillation cells and Rad 7 and the passive measurements were done using Personal Radon Dosimeter systems (PRD). The laboratory based calibration system provided the average activity concentration of 3.40 ± 0.10, 3.25 ± 0.21, 3.59 ± 0.61 Bq l−1 for scintillation cells, Rad 7 and PRD respectively. These systems were subjected to comparison in ambient condition within the facility (mines adit) and the activity concentration of 0.62 ± 0.17, 0.71 ± 0.10 and 0.57 ± 0.04 Bq l−1 has been found for the Alpha Guard, Scintillation cells and PRD respectively. In the field conditions, more fluctuation in the radon profile is anticipated due to site based variations in the ambient / temporal conditions.

Gamma Survey equipment (Target Identifinder) has an in built internal calibration system with a nCi Cs-137 source. The initial calibration and, furthermore, the permanent and continuous stabilization running in the background in parallel to any performed measurement, are based on the said internal source. External calibrations in background region were carried out using K-40 standard and uranium ore source.

5 Result and Discussion

The seasonal variability of radon profile in the atmosphere mostly depends on the geo-genic characteristics and subsequent radon exhalation rate from the surface layer and the horizontal and vertical dispersion phenomena [7]. During the summer maximum exhalation can be observed. The filled pore of the rainy season is unfavourable to the radon exhalation from the surface. Turbulent mixing towards vertical is also quite high in the summer, resulting in a decrease of radon concentration near surface. Contrary to this, vertical exchange is smaller during the winter with frequent inversions. Such reduced mixing far exceeds the anticipated poor exhalation rate of the winter thereby increasing the atmospheric radon concentration during the winter. Overall effect is a maxima of radon during the winter and minima in rainy or summer season. The variation in radon concentration is presented in (Fig. 2). As evident from the box and whisker plot the median concentration is 25–30 Bq m−3 in different directions with respect to the tailings pond. In the box and whisker plot, the lower and lines sides of the rectangle is the 1st and 3rd quartiles, line roughly in the centre of rectangle is the median or 2nd quartile. The two dashed (-) small lines at the top and bottom above the rectangle are rest of the data (extension of the data) and an independent (x) or (-) is statistically an outlier. The central line exactly in the midway of the rectangle is suggestive of a symmetrical normal distribution. The central lines above and below the small rectangle (at the centre of the box) represent asymmetrical nature of the data distribution. Central lines in different boxes are normally used for comparing the median. This can be used even for the non parametric distributions also. There is a narrow gap between 1st and 3rd quartile and the data is extended up to 50 Bq m−3. These levels compare with baseline studies carried out during 2006–2008 with maximum concentration of atmospheric radon in this range (Table 1). These samples were collected as part of baseline compliance study and the range provided is indicative in nature. Also, at a later stage, a number of other factors have been identified which included space and time variables with respect to the facility operations. Only a few outlier has been observed which is also within the baseline range (Table 1) reflecting the absence of the localized effect in the nearby areas. Seasonal variation (Fig. 3) is quite distinct feature and the median radon concentration starts falling after the month of March and lowest during May–June and September. Minima during May–June are due to the vertical turbulent mixing phenomena. Between June end and July beginning initial precipitation is favourable for fast radon exhalation from the surface, resulting in slight increase in the concentration. Once the pores start filling the exhalation process is diminished and minima can be observed during the rainy season of Aug-Sept. In all the four seasons radon measurements have been carried out at sixteen villages up to a distance of 20 kms. The radon profile in the villages was comparable to that from the area in proximity of the tailings pond (Fig. 4). Contribution of the tailings pond is practically insignificant in affecting the existing background of this area.

Fig. 2
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Radon concentration at the bund of the tailings pond

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Table 1 Gamma level and atmospheric radon concentration around Turamdih complex (2006–2008)
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Fig. 3
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Seasonal variation in radon profile around tailings pond

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Fig. 4
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Variation radon conc. in villages around tailings pond

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Gamma level variation around the tailings pond is shown in (Fig. 5). The levels in the close proximity of the tailings (at the edge of the tailings pond and the adjacent road) is slightly elevated as compared to that on the embankment road constructed on eastern and western side. Southern and northern side the gamma level measurements were done on the approach road at the hill, which is at a much higher elevation. Unlike eastern and western side comparison with the edge-road separation is not possible (due to accessibility problem). The variations on this road and road in proximity of the embankment towards the hills are natural, which is within the background fluctuation of 0.1–0.3 µGyh−1 this area. Some of the measurement on the eastern and western side of the embankment was showing bit elevated gamma level (up to 0.40 µGyh−1 on bund wall) to due to proximity with the tailings pond edge. Also, the gamma level variation of the tailings is not uniform due to the variations in the grade of the processed ore and isolated measurements near the edge can be higher. The gamma level in eighteen villages surveyed for ten years has shown a variation of 0.16 ± 0.10 µGy h−1. One way ANOVA also confirmed that there has been no change in the gamma radiation exposure level during the last decades following the discharges at tailings pond. TLD based cumulative gamma survey of the villages confirmed that the average annual gamma exposure is 1.2 ± 0.2 mGy y−1. It is worth mentioning that the area is known for uranium mineralization and wide variation is due to natural geogenic characteristics of this area. The levels are comparable to earlier investigations carried out in the Singhbhum region of Jharkhand with reported cumulative annual average gamma exposure level 1.1 mGy y−1 [8].

Fig. 5
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Variation in gamma level around tailings pond

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Air particulate has been collected from sixteen different locations around the facility. The activity concentration levels were compared with the controlled areas. The results of analyses of the particulate samples were averaged out at 1.7 ± 1.1 mBq m−3. Apart from this, seasonal variability in the alpha activity profile has also been estimated at one fixed location within the campus. The seasonal variability has shown maxima during the summer and a minimum concentration profile during the rainy season (Fig. 6). Observable change could not been seen when the results were compared with earlier investigation carried out during 2006–2008 as part of the baseline studies (Table 2). The levels are far below the reference level of 10 mBq m−3 for the mixed uses. At this level, the worked out background inhalation dose to the public for the respirable size dust particulate assuming an outdoor occupancy of 2000 h y−1, is < 0.1 mSv y−1 [9].

Fig. 6
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Seasonal variation in alpha activity in respirable dust

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Table 2 Long−lived Alpha activity concentration in air around Turamdih complex (2006–2008)
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6 Conclusion

Wide variation in the gamma level and outdoor radon has been observed in areas around the tailings pond. The variations are of the geogenic origin and were not associated with the discharges at the tailings pond. The atmospheric radon concentration profile has been observed to be affected by the seasonal conditions with the minima during the summer and rainy season. No such seasonality could be observed for the gamma level variation or the changes were not observable within the sensitivity range of the system. Particulate activity was originated from the other environmental sources and is not linked to the fine slurry discharges at the tailings pond.